Typical epicardial defibrillation leads are sutured directly to the pericardium or epicardium to direct energy through the heart. The electrodes are sized to maximize the amount of energy delivered to heart tissue without shunting current from lead to lead. The lead configuration used most commonly for epicardial placement is one electrode on the anterior (right ventricle) and one on the posterior (left ventricle) of the heart. Epicardial defibrillation electrodes are commonly oval or rectangular shaped assemblies having a conductive element partially embedded in an insulative backing. An example of this type of electrode is disclosed by Heilman in U.S. Pat. No. 4,291,707 which describes a rectangular mesh electrode having decreased exposed metal at the periphery of the electrode to even the current density across the electrode surface. Another example is presented by Holleman in U.S. Pat. No. 4,817,634 wherein a coil electrode is arranged in the shape of concentric ovals, a spiral oval, or a triangle. Other shapes have been described, such as a shoe, a molar, and a skull in U.S. Pat. No. 4,827,932 to Ideker et al., a pair of shorts in U.S. Pat. No. 5,042,463 to Lekholm, and circles and other shapes in U.S. Pat. Nos. 5,063,932 and 4,938,231 to Dahl et al. and Milijasevic et al., respectively. In the above conventional cases, the nonconductive rim is disclosed as being narrow relative to the size of the electrode, at an estimated two to five millimeters in width, arid is provided for suturing the electrode to the heart.
A subcutaneous patch electrode with at least a 0.5" border is disclosed in U.S. Pat. No. 5,044;374 to Lindemans et al. According to Lindemans et al., provision of an electrode pad which extends substantially beyond the conductive portion of the electrode is believed to reduce the chance that transthoracic defibrillation shocks applied in the vicinity of the subcutaneously implanted electrode will propagate through the electrode to the defibrillator to which it is attached.
It is desirable to reduce the size of an implantable cardioverter/defibrillator (ICD) in order to improve patient comfort, reduce risk of erosion through the skin, and facilitate pectoral placement. Because the batteries and capacitors account for a large portion of the defibrillator, reducing the defibrillation threshold (DFT), or the amount of energy required to defibrillate the heart, is key to allowing the device size to be reduced. Using less energy to defibrillate has the added benefit of improving patient comfort and reducing trauma to the conduction system of the heart.